In original DOCSIS 1.0 systems, a single downstream channel on the hybrid fiber coaxial cable signal delivery system was shared by multiple upstreams. This was necessary because the maximum symbol rate in DOCSIS 1.0 was 1.28 megasymbols per second with QPSK modulation. This so limited the throughput that in order for users to have a good experience, it was necessary to have multiple upstream channels, each of which was coupled to only a percentage of the cable modems of the entire system with all cable modems coupled to the same downstream. In this model, the cable modems all receive video programs or requested broadband services by the same downstream, and the Cable Modem Termination System (CMTS) knows on which upstream to listen for requests, replies and upstream data from each cable modem.
In DOCSIS 1.0 systems, the upstream was minislots which are equivalent to timeslots, and each cable modem received synchronization messages (sync messages) on the downstream which contained timestamps. These timestamps were samples of a timestamp counter in the CMTS which served as the master clock. Each cable modem (CM) synchronized a local timestamp counter to the master timestamp counter in the CMTS using the timestamp samples in the sync messages. The master timestamp counter count was and still is in current DOCSIS systems used to determine the boundaries in time of upstream minislots. The local timestamp counter in each CM was used to determine when each minislot occurred for purposes of timing upstream bursts transmitted by each CM. In DOCSIS systems, each CM is allowed to transmit upstream only during assigned minislots. Assignments come to each CM by downstream messages called grants.
Before a CM can transmit upstream, it must register its presence with the CMTS and be trained. The training process involves both initial and periodic ranging, equalization, power offset adjustment and frequency offset adjustment although equalization was only implemented in DOCSIS 1.1 and later. Ranging is the process of determining the offset between the timstamp counter in the CM and the timestamp counter in the CMTS and setting a proper offset given the distance of the CM from the CMTS. The proper offset is the offset such that when a grant for an upstream burst for minislot 100 is received, the CM can use its local timestamp counter to determine when to transmit and the timing will be such that the burst arrives at the CMTS with its boundaries aligned in time with the boundaries of minislot 100.
Initial ranging is carried out by all CMs by receiving a ranging invitation in a MAP message broadcast in the downstream that identifies an initial ranging window in terms of a group of contiguous minislots in the upstream—an interval during which no modem is assigned to transmit payload data. Initial ranging intervals are put in the MAP messages periodically so new modems which have powered up can get synchronized with the system, and ranging if the first thing they do. They do this by searching the MAP messages sent downstream to find the minislot boundaries of an initial ranging window. Each CM that is training then sets its initial offset (between its timstamp counter and the timestamp counter of the transmitter of the downstream the CM is listening to) so that it appears to be right next to the CMTS. The CM then transmits an initial ranging burst for ranging and equalization and power level training. This initial “station maintenance burst” (hereafter ranging burst) has a preamble of symbols known to the CMTS.
This initial station maintenance burst is transmitted upstream at a time which that CM thinks will cause its ranging burst to arrive during the initial ranging window. The CMTS receives the ranging burst at some time within the window (because the initial ranging window is very wide), and, assuming there is no collision, will be able to measure the time offset of the time of reception of the burst versus the time of the opening of the initial ranging window. If the CM's offset is wrong, the CMTS will detect this fact by detecting the start of burst by detecting the preamble. The CMTS will then measure the offset, and send a message in the downstream addressed to the CM telling it by how much to adjust its offset and in which direction to achieve synchronization.
Periodic ranging happens periodically, and is by invitation only by a message in the downstream directed to a specific CM telling it when a much more narrow periodic ranging window will be open for that CM to send a periodic ranging burst.
The ranging burst preamble is also used to generate equalization coefficients for the upstream channel. These coefficients are sent back down to the CM after convergence in the CMTS where they are convolved with the existing coefficients of the precode filter of the CM upstream transmitter to generate new equalization coefficients for the precode filter which is used to filter subsequent upstream bursts.
With the advent of DOCSIS 2.0 and DOCSIS 1.1 systems described in national standards published by Cable Labs (which are hereby incorporated by reference and cited as prior art), higher upstream symbol rates and more complex constellations have made more upstream throughput available. In fact, the situation is now almost completely reversed in that there is more upstream capacity than downstream. With DOCSIS 2.0, the increased upstream traffic throughput availability often makes it a waste to dedicate a single upstream receiver at the CMTS to each optical node since the CMTS receiver for each optical node is under utilized. It would be advantageous therefor to pool the traffic from multiple upstreams from different optical nodes so the the full upstream capacity of the media and the CMTS receiver is used.
Further, current CMTS configurations for DOCSIS 1.0 and 1.1 installations have a fixed ratio of upstreams and downstreams with one downstream shared by a plurality of upstreams in all the CMTS equipment available in the prior art. Thus, when the demand exceeds the capacity of the single downstream, more equipment must be purchased to add another downstream and that adds 4 to 6 new upstreams which are not needed. Thus, the cable operator has to pay for more equipment than is needed to just add a new downstream channel.
To provide flexibility to cable operators to meet demand for more downstream capacity without forcing them to buy more upstream capacity than they need, a need has arisen for a method of sharing a single upstream between two or more downstreams. This provides maximum utilization of the CMTS equipment and reduces the costs to the cable operator. In other words, it is advantageous to be able to alter the prior art fixed ratio of upstreams to downstreams to fit current needs and to be able to change the ratio in the future as needs change.
Further, the CMTS equipment currently in existence have hard wired relationships between transmitters and receivers and the optical nodes they serve. Thus, when demand changes and more downstreams or upstreams need to be coupled to the same optical node or a plurality of downstreams need to be split up between a plurality of optical nodes, major rewiring at the CMTS is necessary which is laborious and time consuming.
Accordingly, a need has arisen for a method to share an upstream between multiple downstreams and to provide a flexible mapping between different numbers of downstreams and a shared upstream. A need has also arisen for equipment and processes which obviate the re-wiring problem by providing a sort of cross-bar switch mechanism which can flexibly map different numbers of downstreams to a shared upstream and to split out multiple downstreams to multiple optical nodes so that each gets a copy of each of the multiple downstreams. This “crossbar switch” must also be able to divide up multiple downstreams that share an upstream among a plurality of optical nodes. Finally, this equipment must monitor the upstream and downstream equipment and reconfigure to connect new transmitter or receiver equipment to take over for failed equipment of a downstream or upstream.